Orthogonal Frequency-Division Multiplexing (OFDM)
OFDM uses multiple orthogonal sub-carriers to transmit information at a relatively low symbol rate. As an advantage, OFDM can withstand severe changes in channel state and quality, such as high frequency attenuation, narrowband interference, and frequency-selective fading due to multipath, using a single carrier. Channel equalization is simplified because OFDM uses slowly-modulated narrowband signals rather than one rapidly-modulated wideband signal. A low symbol rate enables guard intervals and time-spreading, while eliminating inter-symbol interference (ISI). Some of the subcarriers in some of the OFDM symbols carry pilot signals for estimating the channel state, and performing synchronization.
Orthogonal Frequency Division Multiple Access (OFDMA)
As a disadvantage, OFDM does not provide multi-user channel access to a channel. OFDMA corrects this problem by time, frequency or coding separation, of multiple users. That is, frequency-division multiple access is achieved by assigning different OFDM sub-channels to different users. A sub-channel is a group of subcarriers, which need not be physically contiguous. OFDMA is used in the uplink of the IEEE 802.16 Wireless MAN standard, commonly referred to as WiMAX.
WiMAX
The IEEE 802.16 standard defines an air interface, while WiMAX includes both the IEEE 802.16 air interface and the networking aspect of the system. WiMAX is a broadband wireless access technology, see “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems,” IEEE Computer Society and the IEEE Microwave Theory and Techniques Society, October 2004, and “IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems, Amendment 2; Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands,” IEEE Computer Society and the IEEE Microwave Theory and Techniques Society, February 2006, incorporated herein by reference.
Antenna Selection
According to the IEEE 802.16 standards, multiple antenna elements and radio frequency (RF) chains are supported in base stations (BS) and mobile stations (MS). Due to the high cost of RF chains and relatively low cost of antennas, the number of RF chain (N) is usually less than the number of antennas (M), that is, N≦M . Therefore, in order to transmit/receive signals, the RF chains are connected to selected antennas.
It is known that each antenna provides a different propagation path that experiences a distinct channel gain. Therefore, it is important to selectively connect N of the M antennas to N RF chains so that the transmitting and receiving performance at the BS and the MS is optimized. This function is known as antenna selection (AS). Antenna selection is a method to improve system performance in terms of bit error rate (BER), signal to noise ratio (SNR) and throughput (TH).
Antenna selection is also used by other standards, such as 3GPP Long Term Evolution (LTE), R1-063089, “Low cost training for transmit antenna selection on the uplink,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, R1-063090, “Performance comparison of training schemes for uplink transmit antenna selection,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, R1-063091, “Effects of the switching duration on the performance of the within TTI switching scheme for transmit antenna selection in the uplink,” Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#47, and R1-051398, “Transmit Antenna Selection Techniques for Uplink E-UTRA,” Institute for Infocomm Research (I2R), Mitsubishi Electric, NTT DoCoMo, 3GPP RAN1#43, R1-070524, “Comparison of closed-loop antenna selection with open-loop transmit diversity (antenna switching between TTIs),” Mitsubishi Electric, 3GPP RAN1#47bis, all incorporated herein by reference.
WiMAX Network
FIG. 1 shows a conventional IEEE 802.16 WiMAX network. The network uses a point-to-multipoint communications between BS, and MS. The BS manages and coordinates all communications with the MS1-MS3 in a particular cell on connections 101-103, respectively. Each MS is in direct communication with one BS, and the BS communicates with an infrastructure 110 or “backbone” of the network. All communications to/from the MS must pass through the BS. In order to carry out basic wireless communication, both the BS and the MS are equipped with at least one RF chain. Normally, the number of antenna elements and RF chains is equal at a BS, which implies that N=M. However, given the limitation of cost, size and energy consumption, it is usually true that an MS has more antennas than RF chains. Therefore, antenna selection is used at the MS.
The legacy IEEE 802.16 standard supports both time division duplex (TDD) and frequency division duplex (FDD) modes. The antenna selection describes herein applies to both modes.
Frame Structure
As shown in FIG. 2, the TDD mode uses a frame structure on the uplink from the MS to the BS and the downlink from the BS to the MS. The preamble, FCH, bursts, maps, and gaps TTG and RTG are completely defined in the standard. In FIG. 2, the horizontal axis indicates time, and the vertical axes subchannels. A first subframe is for downlink (DL) transmission, and the second subframe is for the uplink (UL). In both the downlink and the uplink subframes of IEEE 802.16, OFDMA is used for multi-user channel access. OFDMA separates sets of orthogonal subcarriers (sub-channels) in the frequency domain and time slots in the time domain so that multiple MS can share all bandwidth resources, such as time slot and frequency subcarriers. Thus, in contrast with OFDM where only a single user can be accommodated at any one time, OFDMA allows multiple MSs to communicate concurrently in OFDMA system.
In FIG. 1, each connection, such as 101, 102 and 103, between the BS and the MS is allocated a time-frequency resource, which contains a two dimensional block, i.e., time duration and frequency subcarriers, With OFDMA technology, the BS can communicate with all MSs on connection 103, 102, 103 by using two-dimensional bandwidth resource.
In the IEEE 802.16 standard, a minimal resource unit to be allocated is a slot 200. A size of the slot 200 is based on the permutation modes that the MS and the BS use for transmissions in uplink and downlink. A permutation mode defines the type of resource allocation in time and frequency domains. Different modes are defined for the UL and the DL. By using a specific permutation, a given number of OFDMA symbols and subcarriers are included in each slot.
FIG. 3 shows a structure of an OFDMA symbol 300, where Ts is the symbol duration, Tb is the information (data) duration and Tg is the cyclic prefix CP 301. The CP 301 is derived from the data at the end of Tb, which are copied to the beginning of the symbol. Tg is a configurable time period and is approximately a few microseconds long. The frequency subcarriers are generated by a fast Fourier transform (FFT) to construct the complete frequency spectrum. Frequency subcarriers are classified into groups according to different uses, such as DC, data, pilot and guard subcarriers.
The current IEEE 802.16e standard, which uses OFDMA for both downlink and uplink multiple access, does not support antenna selection at mobile stations. Thus, it is desired to provide antenna selection for handsets in OFDMA type of networks. Because the OFDMA, unlike OFDM, allows concurrent access to the channel by multiple users, antenna selection is non-trivial.